Piston rings are well known. They are usually received within an annular groove disposed about an outer periphery of a piston. In turn, the piston is reciprocalable within a cylinder. A piston compresses fluids such as gases within the cylinder. In an internal combustion engine, these fluids are ignited, and expand, forcing the piston away from the point of ignition. Not all of the fluids are ignited, however. Instead, some of the fluids are typically trapped in a crevice defined between an outer radial surface of the piston and the relatively cool cylinder wall, and between the top piston ring and the top of the piston. The trapping of fluids within the crevice lowers combustion efficiency because the trapped fluids do not ignite. Further, when exhausted from the assembly, the trapped fluids increase pollutant emission levels, particularly hydrocarbon emission levels.
Nor are conventional piston rings in total circumferential contact with the walls of a cylinder. Instead, there is an open-end clearance between the piston ring and the cylinder wall that results in the undesirable escape of fluids from the cylinder. Fluids in the crevice may bypass the ring and escape, to be replaced by additional fluids, reducing operational efficiency and increasing undesired pollutant emission.
To reduce the emission of pollutants it is known to position a piston ring near the top of a piston such that an upper surface of the piston ring is exposed to combustion gases. The piston ring includes two legs defining a channel, each leg being received in a groove of the piston. Such prior art piston rings include an intrinsic radial tension intended to force the piston ring into greater contact with the cylinder wall. In fact, it has been taught that the use of a ring without an intrinsic radial tension, a so-called dead ring, will not work correctly in such an orientation. Besides teaching the use of a piston ring with an intrinsic tension, the prior art also teaches that piston rings with an intrinsic tension should include a controllably imbalanced or non-symmetrical cross-section with non-symmetrical grooves to encourage torsional twist.
There are several problems with a piston ring having intrinsic loading and a non-symmetrical cross-section. Intrinsic loading continuously places stresses upon the piston ring. Failure results unless piston rings of a substantial size are used. The continuous loading also favors damaging wear to the ring and to the cylinder wall. Further, the use of torsional twist places only a small portion of the piston ring in facial contact with the combustion wall and the walls of the piston ring grooves, facilitating additional damaging wear to the ring, the walls of the piston ring grooves, and to the cylinder wall.